Preserved skeletal muscle oxidative capacity in older adults despite decreased cardiorespiratory fitness with aging.

KEY POINTS Healthy older adults exhibit lower cardiorespiratory fitness ( VO2peak ) than young in the absence of any age-related difference in skeletal muscle mitochondrial capacity, suggesting central haemodynamics plays a larger role in age-related declines in VO2peak . Total physical activity did not differ by age, but moderate-to-vigorous physical activity was lower in older compared to young adults. Moderate-to-vigorous physical activity is associated with VO2peak and muscle oxidative capacity, but physical inactivity cannot entirely explain the age-related reduction in VO2peak . ABSTRACT Declining fitness ( VO2peak ) is a hallmark of ageing and believed to arise from decreased oxygen delivery and reduced muscle oxidative capacity. Physical activity is a modifiable lifestyle factor that is critical when evaluating the effects of age on parameters of fitness and energy metabolism. The objective was to evaluate the effects of age and sex on VO2peak , muscle mitochondrial physiology, and physical activity in young and older adults. An additional objective was to assess the contribution of skeletal muscle oxidative capacity to age-related reductions in VO2peak and determine if age-related variation in VO2peak and muscle oxidative capacity could be explained on the basis of physical activity levels. In 23 young and 52 older men and women measurements were made of VO2peak , mitochondrial physiology in permeabilized muscle fibres, and free-living physical activity by accelerometry. Regression analyses were used to evaluate associations between age and VO2peak , mitochondrial function, and physical activity. Significant age-related reductions were observed for VO2peak (P < 0.001), but not muscle mitochondrial capacity. Total daily step counts did not decrease with age, but older adults showed lower moderate-to-vigorous physical activity, which was associated with VO2peak (R2  = 0.323, P < 0.001) and muscle oxidative capacity (R2  = 0.086, P = 0.011). After adjusting for sex and physical activity, age was negatively associated with VO2peak but not muscle oxidative capacity. Healthy older adults exhibit lower VO2peak but preserved mitochondrial capacity compared to young. Physical activity, particularly moderate-to-vigorous, is a key factor in observed age-related changes in fitness and muscle oxidative capacity, but cannot entirely explain the age-related reduction in VO2peak .